THE number of children contracting whooping cough is increasing, with the disease commonly affecting young infants who suffer a severe illness.

There is strong evidence to suggest that waning immunity among older children and adults, who were vaccinated against the disease, may be causing the increase.

Whooping cough (pertussis) is highly contagious and the Irish figures are in line with international trends.

Young children who get whooping cough can require a prolonged spell in hospital, according to a new study by doctors at Temple St Children's Hospital in Dublin.

They studied a number of admissions to the hospital.

"Reducing transmission from known infected patients still plays a vital role in controlling the spread of disease," they reported in the Irish Medical Journal.

The number of cases notified in Ireland decreased in the early 2000s. Between 2003 and 2008, 40 to 104 cases were notified annually with infants suffering the highest incidence of illness and death.

"Figures doubled from 2010 to 2011 and doubled again from 2011 to 2012," reported the Temple Street study.

There were 18 laboratory confirmed cases diagnosed at Temple St and 15 of these were infants under six months. The infants with whooping cough ranged from 36 to 96 days old and had a median age of 44 days.

Ten infants had documented exposure to a sick member of their family - most commonly the child's mother or older siblings with similar respiratory symptoms, often with prolonged cough.

"Patients presented to the hospital as a result of increased severity of symptoms," reported the study. They spent from four days to 13 days in hospital. "Calculating for a cost of €800 for a night on a general ward and €1600 for a night in intensive care, the total cost of admissions to the hospital was in excess of €90,000, with an average cost of €6,450 per patient.

The disease is known for uncontrollable, violent coughing which can make it difficult to breathe. A bout of coughing is followed by a need to take in a deep breath which results in a 'whooping' sound. The disease can be fatal, especially in babies under 12 months. Immunisation is the best prevention strategy.

The whooping cough vaccine is currently given in Ireland as part of the 6-in-1 vaccine. Apart from whooping cough, this vaccine protects against diphtheria, tetanus, Haemophilus influenzae type b, hepatitis B and polio. Three doses of the 6-in-1 vaccine are usually given at two, four and six months of age. A fourth dose is recommended at four or five years.

Getting vaccinated while pregnant may help to protect the unborn baby from developing whooping cough in the first few weeks of life as the immunity is passed from mother to baby.